This invention relates to plastic lids of the flexible type which are used to close commodity containers, such as plastic ice cream pails. Such lids are normally stacked together vertically in the course of handling and container filling operations. Automatic filling machines are in use which mechanically separate the lids one at a time, and automatically place them on containers which have been filled. The stacked lids tend to stick together as the result of a suction effect between adjacent lids. This problem necessarily interrupts and slows down the high speed operation of automatic filling machines.
Lids of the type to which the improved lid structure of this invention is directed are disclosed in U.S. Pat. No. 3,880,288 issued to Carl T. Hunter. Such plastic lids include a raised locating ring on the top wall of the lid which functions in combination with an adjacent shoulder on the top wall to permit convenient nesting and stacking of the lids prior to their placement on filled containers. The Hunter patent, as well as U.S. Pat. No. 4,111,329 issued to John Edward Lampman, disclose a locking bead or rim formed on the inside surface of a side wall or skirt of a plastic lid for use in securing the lid in tight engagement with a coacting rim or bead on the top of a container. The aforesaid Lampman patent discloses as stacking means elongated, vertical ribs formed on the outer surface of the lid side wall.
Efforts have been made to overcome the problem of plastic lids sticking together in stacks. U.S. Pat. No. 3,373,896 issued to Paul Davis discloses the use of cam surfaces on portions of plastic lids to assist in mechanically separating the lids one from another in a stack. U.S. Pat. No. 3,724,710 also issued to Paul Davis is more pertinent in that it discloses the use of depressions formed in the mating portions of adjacent stacked lids to define air passages for communicating the region between stacked lids with the atmosphere.
With the aforesaid prior art lid structure and lid sticking problems in mind, we have developed an improved lid construction which is particularly effective in facilitating the separation of stacked, plastic lids of conventional, stacking ring and stacking shoulder design.